1. Field of the Invention
The present invention relates generally to a fiber optic receiving circuit and, more particularly to a fiber optic receiving module for converting an optical signal to an electrical signal by means of an optical fiber.
2. Description of the Related Art
A conventional fiber optic receiving circuit includes an amplifier of parallel feedback type as shown in FIG. 1. This is disclosed in Published Unexamined Japanese Patent Application No. 60-134611. Such a fiber optic receiving circuit includes an input terminal 2, an amplifier 1 having a negative-phase output terminal 3, a photo diode 4 connected between the input terminal 2 and a power source Vcc, and a feedback resistor 5 connected between the input terminal 2 and the negative-phase output terminal 3.
The operation of the above circuit can be logically expressed as follows: EQU v.sub.o =-Av.sub.i ( 1)
wherein an input voltage of amplifier 1 is v.sub.i, an output voltage is v.sub.o, and an open loop gain is -A. ##EQU1## wherein a signal current supplied from the photo diode 4 is i, resistance of the feedback resistor 5 is R.sub.F, and a junction capacity of the photo diode 4 is C.sub.J.
If v.sub.i is eliminated from equations (1) and (2), ##EQU2## normally A&gt;&gt;1, and if the following are provided, ##EQU3## Therefore, a cut-off frequency f.sub.c in the area of -3 dB down can be obtained as follows: ##EQU4##
Also, the conversion gain at the frequency which is sufficiently lower than f.sub.c becomes R.sub.F. Therefore, to obtain sufficient gain over a broad range of frequency band, it is required that R.sub.F and A are set to be extremely large and C.sub.J is set to be extremely small.
However, there is a case that junction capacity C.sub.J of the photo diode 4 cannot be made small. For example, in a case where a light receiving area must be enlarged in connection with the optical fiber to be used, or the photo diode 4 must be formed of not PIN structure but PN structure, a problem exists in that a sufficient frequency band cannot be obtained with the gain to be used.